Setup a test environment similar to your production setup, and try upgrading and see what happens?
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ZoredacheMay 7 '12 at 5:31

We don't have a second machine where I can build debian with vservers. When I upgrade one vserver (only) I didn't started vserver. I cannot try upgrade only main machine because this can have 'epic fail'.
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Bartosz KowalczykMay 7 '12 at 5:50

First of all make sure your kernel is 2.6.32. If it is not, it happened to me that auto-upgrade failed for some reason (Because you are running a BPO kernel for example). In this case, depending on your architecture :

aptitude install linux-image-2.6-vserver-amd64

Given that the above kernel is used, you may proceed with the system upgrade. In similar situations i preffered to upgrade VMs first and then the host itself.

For the VMs the procedure would be:

Update the /etc/apt/sources.list for squeeze

Do the APT update and download the packages (especially when doing this from the internet, assuming there is no local repository mirror):

sudo apt-get clean && sudo aptitude update

sudo aptitude full-upgrade --download-only

Make sure that you install the appropriate driver packages for your hardware. It would be unpleasant to find yourself cut off from your remote server :)
Example: sudo aptitude install firmware-linux-nonfree

Do the safe upgrade for a few times until there is nothing left:
sudo aptitude safe-upgrade

Finally do a full upgrade:
sudo aptitude full-upgrade

For the host of your virtual machines there will be an additional step in updating GRUB to GRUB2. You can check if your bootloader was installed properly by running:
update-grub
If you do not see any errors, it is most likely safe to reboot.

If you get ANY error message about missing drivers, DO NOT REBOOT. Install necessary drivers before rebooting.

Depending on your actual environment and setup the above steps might differ a little, but these should give you a general idea.